The disclosures herein relate generally to electromagnetic shielding and heat dissipation and more particularly to an integrated system for providing both electromagnetic shielding and heat dissipation for integrated circuits in a portable computer system.
A portable computer is a self-contained personal computer which can be easily moved to and operated at various locations. Portable computers are often referred to as laptop or notebook computers. To be portable, these computers must be small, compact, and lightweight. The conventional portable computer includes a base portion and a lid portion that pivotally opens from the base portion when the portable computer is in use. The lid portion contains a flat panel display such as a liquid crystal display (LCD) or other relatively small display.
Notebook computers incorporate electromagnetic shielding to reduce electromagnetic emissions in order to meet regulatory standards on such emissions. Portable computers also use thermal transfer mechanisms (heat-sinks or heat spreaders) to cool the CPU which accounts for almost half the power dissipated within the computer system.
An electromagnetic interference (EMI) shield is typically a metallic partition placed between two regions of space. The EMI shield controls the propagation of electric and magnetic fields from one of the regions to the other. An EMI shield may be used to contain electromagnetic fields if the shield surrounds the source of the electromagnetic fields. Many different sources of EMI noise are present in today's computer systems. Typically several mechanisms contribute to the total radiated EMI emissions from a computer system. Within a typical multi-layer Printed Wiring Board (PWB), circuit traces, internal power planes, and the integrated circuits attached to the PWB all have some contributory effect on radiated EMI emissions.
Between typical multi-layer PWBs exists another source of a computer system's total radiated EMI emissions, in which two different PWB structures at two different radio frequency (RF) potentials or different RF phase angles create a coplanar dipole antenna arrangement. The two or more PWBs radiate RF energy with respect to one another, creating multiple lobe electromagnetic waves into surrounding space. The RF voltage source which feeds each of the PWBs with the energy to radiate, usually exists in RF voltage losses in the connector, by which they are interconnected.
A solid EMI shield that completely surrounds a product can be at any potential and still provide effective EMI shielding. That is, the shield prevents outside influences from affecting circuits inside the EMI shield and vice versa. Thus, the EMI shield need not be grounded or have its voltage potential defined in any way. However, an ungrounded or undefined EMI shield should completely enclose the object being protected and that object being protected should have no connection to the outside world.
In practice, however, the EMI shield is not a complete enclosure, and the object inside does have connections to the outside world, either directly, through signal and/or power leads, or indirectly, through stray capacitance due to holes in the EMI shield. In such cases, the EMI shield should be grounded or have its voltage potential defined with respect to the noise source to prevent the noise source's potential from coupling to the enclosed object. An ungrounded or undefined EMI shield's potential varies with conditions and location, and therefore the noise coupled to the object inside also varies.
Grounding also has other benefits. Grounding provides a path for RF currents to flow on the structure. Grounding also prevents the buildup of AC potentials on the equipment enclosure. Grounding provides a fault-current return path to protect personnel from shock hazards. Grounding also prevents the buildup of static charge.
The EMI shield should have a low-impedance coupling with a voltage reference such as a ground plane of a printed circuit board in at least two places in order to properly define the voltage potential or ground the EMI shield in a computer system. However, today's computer systems include high frequency EMI sources such as processors which may require the EMI shield to be electrically coupled to a voltage reference, such as a ground plane, at several locations. The higher frequencies of these EMI sources require closer spacings between the grounding connections of the EMI shield to the voltage reference in order to provide effective EMI shielding. The spacing of these grounding connections is directly related to the desired shielding effectiveness and the upper frequency limit of the shield. One embodiment encompasses the legal requirements of the upper frequency limit of the shield to be 2000 MHz (two billion cycles per second), and to have a shielding attenuation of greater than 40 decibels (dB). These criteria are used to determine the optimum spacing of each grounding embodiment by calculation. Coupling a generally planar EMI shield at several closely spaced locations around its perimeter allows an EMI shield to form the top portion of an effective EMI shield enclosure with a ground plane forming the bottom portion. As with the PWB grounding connections, the spacing of the top perimeter connections is directly related to the desired shielding effectiveness and the upper frequency limit of the shield.
EMI shields constructed from an electrically conductive metal with a high yield strength give the shield resiliency in making electrical contact with an electrically conductive surface electrically coupled to a voltage reference such as a ground plane of a printed circuit board.
Heat distribution has also been a problem with some computers, especially with portable computers. In the past, heat spreaders, heat sinks, heat pipes, and fans have been used to address this problem. One type of heat spreader is a metal piece that is thermally coupled to a processor and distributes heat away from the processor. Typically, a heat sink is made of relatively pure aluminum for good thermal conductivity and for reduced weight. However, aluminum oxide coatings typically form on the outside of items made of aluminum which reduces the ability of the object to provide low impedance electrical connections with other items in contact with the aluminum material. Because heat spreaders typically serve only one function, heat spreaders add extra pieces to the computer system assembly as well as increasing the complexity of the build and repair operations.
Fans and heat sinks provide a cost effective mechanism for thermally managing many types of portable computer systems. Fans, however, require power and heat sinks require space. While power and space are generally in abundant supply in desktop-type minicomputers, portable computers have only a limited supply of both power and space. A commercial advantage is achieved by manufacturing portable computers that are both small and lightweight. Further, portable computers must operate with power conservation in mind. An operable fan may unduly draw upon the batteries of a laptop making it unattractive for long periods of battery-operated use.
Heat pipes are self contained, phase transformation, heat carrying devices, i.e. a superconductor of heat. A typical heat pipe may comprise a closed copper tube having a partial vacuum internally. Water in a hot portion of the tube boils at a lower than usual temperature in the partial vacuum. The boiling water seeks a cooler spot and thus steam moves to carry heat to the cooler spot where the steam condenses to cooler water which returns to the hot spot. The cycle is ongoing which provides a contained circulating system.
Therefore, what is needed is a hybrid cooling device utilizing both an active and a passive cooling method integrated with an EMI shielding device which utilizes the benefits and avoids limitations of each and, in addition, provides enhanced heat dissipation while providing effective EMI containment of an electrical component within a computer system.